Rear Brake Won't Pressurize...help!  By  chris9183 (Read 3,831 times)

Okay, the new master cylinder got here and I attached it and it's definitely pushing fluid through the line faster and more smoothly...but STILL no pressure building in the system no matter what I do or how long I do it. Not one tiny bit.

So I'm now ordering a new rear caliper.

Also, how likely is it to be the actual lines causing the problem? I've not noticed any fluid coming from the lines anywhere...no mysterious drops of liquid anywhere under the bike.

Well, I do believe you have my scooter's brother... LOL!

Your experience is exactly parallel to mine. You won't be sorry about replacing the rear caliper along with the master cylinder. After the old caliper's pistons have been extended WAY far out, I'm uncomfortable with attempting to push them all the way back in. With exposure to the elements, I think it's asking for a leak at the pistons... Some mechanics say it's OK to do that on a car, and some don't. And these scooters aren't as well made as most cars... LOL!

When the new caliper arrives, please do set it over the disk, and see if it has all that extra space (about 1/8") between the pads and the disk. If so, I strongly recommend you remove a pad from the OLD caliper, and grind off all the friction material, and place it BEHIND the inside pad on the NEW caliper, as a spacer. That is what I did, and it immediately solved the problem. (If you do that, and the caliper is too tight over the disk, just grind off a few thousandths-inch of the steel on the "spacer"). You don't want the new caliper pistons to have to extend nearly to their limit just to begin braking action.

If you follow the instruction-video from Alleyoop, you can't go wrong on the back-flush/fill operation. You do need a big syringe like the video shows. You can actually watch the master cylinder fill, and start and stop the fill at will without letting air in, by just closing the bleeder while maintaining pressure on the syringe. With the scooter on the center-stand, and the bars turned slightly right, the master-cylinder is nearly level, and easy to see and fill.

Once the system is full, you should have operating brakes immediately. (That may ONLY be true with the spacer in place.) If you just install the new caliper, and it has a lot of free-space, it will have to pressurize and push the pistons out, which may require more fluid to be added.

In addition, if your rear caliper is like mine, once the new pads wear just a little, you'll be right back to no rear brake again.

It's unlikely that your line is bad. I too questioned my lines, but as with yours, I saw no leaks, and now with the new caliper, all is fine. If, after all is working, you start to lose pressure, the line might be leaking. But... that is not too likely.

Do be sure to use DOT 3, or DOT 4 brake fluid. That's what these use. Also, the fluid is hard on paint! So... Stuff a big old towel all under the master-cylinder to absorb any fluid you may "over-squirt"...

I have noticed that the rear 2-piston caliper has a little different feel from the front single-piston caliper. The lever moves a little further, but everything works fine. Mine was always that way, and the new caliper is the same.

All this is a whole lot easier to do than it sounds when written out... REALLY... Removing and replacing the pads simply requires removal of the two long allen-bolts they ride on. The pads are under slight spring-pressure and need to be pushed upward to free the bolts, but it's self-explanitory when you look at the caliper.

I think you and I are "pioneers" in this weird rear-brake phenomenon, but as more riders get some miles on their rides (mine has 3,500 miles) the problem will become more common. I honestly feel that many riders get SO fed up with the never-ending problems with these, that they sell or trade off their Chinese scoots before the rear brake wears enough to cause this problem. I dunno, they ARE a real pain, requiring a lot of "wrenching" but for the money they're still hard to beat. I've ridden mine now for FIVE years, with the last FOUR being fairly trouble-free. Now, it's caught up with me... HeHeHe... However, even with repairs, I don't have quite a thousand-bucks invested.

Any help I can give, please let me know...

Leo in Texas

I like you, you're full of energy in your posts, lol.

When the new caliper arrives, the first thing I'll do is check how much space is between the pads and the disk. If there's a lot of space, I'll use the old pads as a spacer.

One thing though, I am using synthetic brake fluid. Is this bad? I did a lot of reading before starting this brake project and I never saw anything about not using synthetic on these scooters, but that doesn't mean I didn't just miss it.

Got to agree with Rockynv... Most riders don't recommend synthetic because of its tendency to trap air. I think when you install your new caliper, it would be best to flush the current fluid out and totally refill with DOT 3 or 4. It's very inexpensive, and these systems don't hold much.

I know many people prefer the vacuum bleeders, and they work well. After watching Alleyoop's video on using a large syringe and reverse-flushing and filling from the bleeder though, I found it REALLY easy and efficient. Everybody has their favorite methods. I just like the ability to do it all by myself, being able to see the fluid level rise and check for absence of air bubbles. If you fill the reservoir, and there still are air bubbles appearing, just "sop up" the fluid in the reservoir and keep on filling until the fluid is bubble-free. It took me about three full reservoirs until all air was gone.

Thanks for thinking I seem full of energy in ANYTHING at my age... LOL!

Please remember, if you use one of the old pads as a spacer, you MUST grind off all the friction material so it's just a smooth steel "biscuit". Any material left on the pad will make it too thick, and could make it apply unevenly. You can easily remove the friction material with a disk-grinder or a drill with a coarse sanding disk. Just clamp the pad in a vice and carefully grind off the old material. Then be SURE the "spacer" goes between the caliper pistons and the new inner (moving) pad to maintain alignment of the caliper with the disk. You only want to "close the gap" so the pistons don't move more than necessary. You're just moving the inner pad closer to the disk.

You'll be back riding in no time!

Today, I finished up my brake fix, and re-assembled all the plastics. Now, I need to finish fabbing up a new exhaust/muffler and I'll be back on the road.

Ride safe!

Leo

Alrighty then, point taken. When the new caliper arrives, I'll drain the system and put in regular brake fluid instead of synthetic. Thanks. I bought synthetic because I just figured if synthetic oil is better, synthetic brake fluid must be better too, lol. I guess it's not the same though.

Here is the exact caliper I got... From eBay seller gy6racingteam. Item number:171019690285 SCOOTER 150CC 125CC GY6 CHROME REAR BRAKE CALIPER PADS $44. with free shipping.

The seller has extensive positive feedback, and my parts came in 5 days. This particular caliper comes with mount, and pads, and both the caliper and mount are chromed. I went with this seller because the assembly was five-bucks cheaper than any others I found, not because of the "bling"... LOL! The chrome is nice-looking though. The caliper went right on, direct-replacement, no problems (except for that GI-NORMOUS, HUGE GAP between the pads... LOL) Which I filled with the home-made spacer...

He sells a lot of parts, and has a good rep.

You might want to check him out.

About the mirror... How about replacing both mirrors with "bar-end" bicycle mirrors? They look great, and a lot of custom scoots use them. There must be an easier way to replace with after-market mirrors than buying a whole new master cylinder, and going through all the fluid-replacement process. I'd even try filling the old threaded hole with JB-Weld, then coating a proper-sized bolt with thin grease, placing it in the hole. Then, when the JB-Weld cures, the bolt should unscrew, leaving fresh threads. But then, I'll try ANYTHING... HeHeHe... It's worth a try...

When does it end? Ah, er, well, I'm not sure it EVER does with these scoots. Even the high-buck ones from Europe and Taiwan sometimes have similar issues. Just part of the game I guess...

As "Red Green" says: "We're pullin' for ya'... after all, we're all in this together!"

Leo (pullin' for ya') in Texas